1. Field of the Invention
The present invention relates to matrix circuits using thin film semiconductors, and more particularly, to matrix circuits in read or display devices.
2. Description of the Prior Art
Recently, two-dimensional liquid crystal displays and strip image sensors for facsimile devices have been constructed using thin film semiconductors of amorphous silicon hydride (a-Si:H) or sintered cadmium sulfide cadmium selenide (CdS-CdSe).
A thin film semiconductor can be deposited readily onto a transparent substrate by glow discharge, reactive sputtering or evaporation methods. It can then be processed into an array of photodiodes, photoconductive type photosensors, field effect transistors, or the like. Thus, inexpensive strip, large-area read or display devices can be formed which have not been realized with conventional crystal semiconductors.
Usually, matrix circuits are used in read and display devices for the purpose of simplification and two-dimensional formation of the circuits. The case of strip image sensors will be considered and the matrix circuits thereof will be described.
FIGS. 1 and 2 show matrix circuits of strip image sensors. Both in FIGS. 1 and 2, n (n=4 in FIGS. 1 and 2) unit elements e of a thin film semiconductor are connected so as to form a block. M such blocks are connected to form an array. For the sake of description, a unit element will be described hereinafter as e.sub.ij where the subscript i is the number of the block to which the unit element belongs and the subscript j is the number of the order of the unit element in the block. Thus 1 =&lt;i =&lt;m, and 1 =&lt;j =&lt;n.
In FIG. 1, the terminals on one side of the unit elements in each block are connected together. The terminals on the other side of unit elements (e.sub.i1 -e.sub.i4 ; 1 .ltoreq.i .ltoreq.4) having the same subscript j in the respective blocks are connected to a corresponding one of lines l.sub.1 -l.sub.4 which are in turn connected to corresponding arrays of switches 1-4 and are selectively connected to ground or an input terminal of an amplifier 5 depending upon the operation of switches 1-4.
Respective unit elements e in each block are impressed with a voltage Vi (i is a block number such that 1.ltoreq.i .ltoreq.m) at which time block unit elements e.sub.il -e.sub.i4 become active. Since an image sensor is used here as an example, respective electric currents, corresponding to the intensity of incident rays, flow through corresponding active unit elements e.sub.il-e.sub.i4. The currents are sequentially input to and amplified by amplifier 5 depending upon the operation of the switch array. Voltages V.sub.1 -V.sub.m are sequentially applied, as shown in the timing chart of FIG. 3, so that currents flowing through all of the elements e corresponding to the intensity of incident rays are sequentially input to amplifier 5.
In the matrix circuit of FIG. 2, lines I.sub.1 -I.sub.4 are connected to the respective inputs of amplifiers 6, the outputs of which are connected to the respective inputs of allotment areas of a shift register 10 which outputs its storage contents sequentially as a time series signal each time a shift pulse (not shown) is input thereto. Since voltages V.sub.1 -V.sub.m are applied as shown in FIG. 3, currents corresponding to the intensity of incident rays are obtained from all of the unit elements e of the array in the matrix of FIG. 2 as in the matrix circuit of FIG. 1.
In order to end the operation of the entire array of units elements e.sub.ij in T.sub.a seconds, each unit element e.sub.ij must come into a normal operational state at latest T.sub.a /m seconds by calculation after the voltage V.sub.i is applied. For example, when T.sub.a -10 msec, and m =64, T.sub.a /m =156 .mu.sec. Although each unit element is given T.sub.a /m =156 .mu.sec by calculation, it can actually only have a leeway of about 50 .mu.sec due to various restrictions.
FIGS. 4A-4C each are graphs of current, as a function of time, flowing through a 10 .mu.m-long gapped coplanar photoelectric type photosensor having an ohmic contact electrode at an n.sup.+layer as a unit element e.sub.ij directly after the photosensor is impressed with a voltage of 10 V. In the graphs, the axis of abscissas denotes time (.mu.sec) and the axis of ordinates denotes current (A).
FIGS. 4A, 4B and 4C relate to the current-time relationship at illuminances of 100(1x), 10(1x) and darkness, respectively.
As is obvious from these graphs, a great current flows directly after application of the voltage of 10 V, but after elapse of 200 .mu.sec, the current subsides to a steady state. In the cases of 10(1x) (FIG. 4B) and the dark state (FIG. 4C), the current, flowing directly after the voltage application, is considerably large compared with the steady-state current. Thus, it will be understood that in the steady-state, the current in the case of 100 (1.times.) is about 5 times as large as that in the case of 10(1.times.) whereas directly after the voltage application, the former current is only 2.3 times as large as the latter current, thereby rendering it difficult to discriminate between the light intensities. It follows that the conventional image sensors are likely to read documents erroneously.
In order to eliminate these problems, a method has been considered which includes the steps of either prolonging the operating time T.sub.a of the entire array of unit elements e.sub.ij or increasing the number n of unit elements and increasing the number of switches 1-4 in FIG. 1 or increasing the number of amplifiers 6-9 in FIG. 2.
However, such a method lowers the performance of the image sensors as a device and raises the cost of the sensors. Thus, such a method is not acceptable.